This provides something that gets discussed regularly: stable
device IDs.  Straightforward; most of it was already present:

    - Moves "bus_name" out of HCD framework into usb_bus.
      This accounts for _by far the bulk_ of this patch, since
      every use of that original field needed to change.

    - Make Vojtech's usb_make_path() use bus_name instead
      of unstable bus numbers ... and usb_device->devpath
      instead of recomputing that same info.  Combine two
      stable IDs, and the result is still stable.  And since both
      are precomputed, usb_make_path() turns into only an
      error check wrapped around snprintf() ... so I inlined it.

    - Minor tweak to usb_device->devpath construction.  It
      still uses "/" for the root hub (better suggestions?) but
      Vojtech wanted "." for separators, so I changed that.

    - The older HCDs (uhci.c etc) initialize usb_bus.bus_name,
      as well as the sharable HCD framework

    - HCD framework no longer uses "bus" member.

These IDs are currently exposed in "input" hotplugging, and
my next patch addresses that for USB network devices.